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Comportement chimique du protactinium(V) en préscence d'ions sulphates

Abstract : Protactinium is experiencing a renewal of interest in the frame of long-term energy production. Modelling the behaviour of this element in the geosphere requires thermodynamic and structural data relevant to environmental conditions. Now deep clayey formation are considered for the disposal of radioactive waste and high values of natural sulphate contents have been determined in pore water in equilibrium with clay surface. Because of its tendency to polymerisation, hydrolysis and sorption on all solid supports, the equilibria constants relative to monomer species were determined at tracer scale (ca. 10 - 12 M) with 233Pa. The complexation constants of Pa(V) and sulphate ions were calculated starting from a systematic study of the apparent distribution coefficient D in the system TTA/Toluene/H2O/Na2SO4/HClO4/NaClO4 and as a function of ionic strength, temperature, free sulphate, protons and chelatant concentration. First of all, the interaction between free species H+, SO42-, Na+ leads to the formation of HSO4- and NaSO4- , for which concentrations depend upon the related thermodynamic constants. For this purpose a computer code was developed in order to determine all free species concentration. This iterative code takes into account the influence of temperature and ionic strength (sit modelling) on thermodynamic constants. The direct measure of Pa(V) in the organic and aqueous phase by g-spectrometry had conducted to estimate the apparent distribution coefficient D as function of free sulphate ions. Complexation constants have been determined after a mathematical treatment of D. The extrapolation of these constants at zero ionic strength have been realized by sit modelling at different temperatures. Besides, enthalpy and entropy values were calculated. Parallely, the structural study of Pa(V) was performed using 231Pa. xanes and exafs spectra show unambiguously the absence of the trans dioxo bond that characterizes the other early actinide elements like U and Np. In concentrated sulphuric acid (12 M), Pa(V) is proved to exhibit a single oxo bond. In 0.5 M and 0.05 M HF medium, the results indicate the absence of any oxo bond : Pa(V) exists in the form of a pure fluoro–complex.
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M.V. Di Giandomenico. Comportement chimique du protactinium(V) en préscence d'ions sulphates. Autre. Université Paris Sud - Paris XI, 2007. Français. ⟨tel-00264952⟩

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